1. Field of the Invention
The present invention relates to a calibration tool for fabricating disk springs, the tool including a circular cylindrical tool body having an insertion bevel at its face end portion for receiving and centering a workpiece within an inner circumferential enveloping surface of the tool body, and having a tool surface at the inner circumferential enveloping surface for aligning the workpiece relative to a forming tool disposed within the inner circumferential enveloping surface.
2. Description of the Related Art
Disk springs of that genus are provided, e.g., for application in friction clutches of motor vehicles, wherein they provide the contact pressure required for pressing the friction lining against a flywheel at the torque output of an exemplary internal combustion engine of a motor vehicle. Such disk springs are made in a forming process from a blank provided in the shape of a circular disk.
In order to prevent the occurrence of speed variations in the drive train of a motor vehicle configured with such a friction clutch, it is important for the fabricated disk spring to have a symmetrical structure, so that it can, e.g., not happen that the circumferentially spaced tongues of the disk spring impart upon the friction liners or upon the flywheel normal force components that differ significantly from one another.
Thus, the fabrication of disk springs is typically completely automated, so that a blank is inserted into the die cavity of a forming tool by a handling apparatus, and then the blank is formed into a disk spring through the association of pressure and heat in the forming tool.
In order to prevent the speed variations recited above, the blank has to be centered relative to the die cavity or relative to the forming dies disposed thereabout. For that purpose, the forming tool includes a circular cylindrical tool body that includes an insertion bevel at one face end portion for receiving and centering the workpiece within an inner circumferential enveloping surface of the tool body. The handling apparatus transports the blank through the insertion bevel into the die cavity, so that the forming tool can close subsequently in order to form the blank in the die cavity.
An important component of the forming tool is thus the circular cylindrical tool body recited above, which includes a tool surface at its inner circumferential enveloping surface, which tool surface is used for aligning the workpiece relative to the forming tool disposed within the circumferential enveloping surface.
The tool configured with the tool surface at the inner circumferential surface is used for centering or calibrating the workpiece relative to the forming tool, and is therefore subsequently designated as calibration tool or calibration ring.
A known calibration tool illustrated in FIG. 1 of the attached drawing includes the insertion bevel recited above at its upper face end portion, through which the blank to be formed into a disk spring is inserted into the die cavity between an upper die and a lower die and can be centered there.
The lower end of the insertion bevel viewed in the axial longitudinal direction includes an inner diameter, which is equal to the inner diameter of the first die portion subsequent to the insertion bevel. In that first die portion, thus the tool surface includes a straight inner circumferential enveloping surface, viewed in that axial longitudinal direction of the calibration tool, which is used for centering the workpiece during approximately the first half of the operating movement of the upper die relative to the lower die of the forming tool.
The first shaped portion of the known calibration tool shall therefore be used to prevent a radial displacement of the workpiece relative to a support surface configured at the lower die during the feed movement of the upper die relative to the lower die.
It has now become apparent, that grinding- or scratching marks occur at that first shaped portion of the known calibration tool, which originate from a physical contact of the blank and the calibration tool in the first shaped portion. The physical contact between the blank and the first shaped portion of the known calibration tool therefore leads to a normal force loading of the outer circumferential enveloping surface of the blank towards the center of the blank, and thus leads to an upset.
Thereby, internal stress is induced in the blank which leads to a warping of the blank, and thus to a nonsymmetrical disk spring with reference to the axis of symmetry extending through the center of the blank. The spring force of disk spring tongues disposed in such a portion can therefore differ from the spring force of disk spring tongues disposed outside of that portion.
It is therefore an object of the present invention to provide a calibration tool, which avoids the forming of such upset or warped portions of the disk spring. Furthermore, a forming tool is to be provided that is configured with the calibration tool in accordance with the invention.